In a Universal Mobile Telecommunications System (UMTS), such as that proposed for the next of the third generation partnership project (3GPP2) standards for the UMTS Terrestrial Radio Access Network (UTRAN), such as wideband code division multiple access (WCDMA) or cdma2000 for example, user equipment (UE) such as a mobile station (MS) communicates with any one or more of a plurality of base station subsystems (BSSs) dispersed in a geographic region. The mobile station is typically a cellular communication device. Each BSS continuously transmits a downlink physical control (pilot) channel signal having the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. Hereinafter, a pilot signal of a BSS will be simply referred to as a pilot. The MS monitors the pilots and measures the received energy of the pilots.
In the WCDMA system, there are a number of states and channels for communications between the MS and the BSS. For example, in the Mobile Station Control on the Traffic State, the BSS communicates with the MS over a Forward Traffic Channel in a forward link and the MS communicates with the BSS over a Reverse Traffic Channel in a reverse link. During a call, the MS must constantly monitor and maintain four sets of pilots. The four sets of pilots are collectively referred to as the Pilot Set and include an Active Set, a Candidate Set, a Neighbor Set, and a Remaining Set.
The Active Set includes pilots associated with the Forward Traffic Channel assigned to the MS. This set is active in that the pilots associated with this set are all within soft handoff range of the MS. The Candidate Set includes pilots that are not currently in the Active Set but have been received by the MS with sufficient strength to indicate that an associated Forward Traffic Channel could be successfully demodulated. The Neighbor Set includes pilots that are not currently in the Active Set or Candidate Set but are likely candidates for handoff. The Remaining Set includes all possible pilots in the current system on the current WCDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
Typically, a BSS services a coverage area that is divided up into multiple sectors. In turn, each sector is serviced by one or more of multiple base transceiver stations (BTSs) included in the BSS. When the MS is serviced by a first BTS, the MS constantly searches pilot channels of neighboring BTSs for a pilot that is sufficiently stronger than a threshold value. The MS signals this event to the first, serving BTS using a Pilot Strength Measurement Message. As the MS moves from a first sector serviced by a first BTS to a second sector serviced by a second BTS, the communication system promotes certain pilots from the Candidate Set to the Active Set and from the Neighbor Set to the Candidate Set. The serving BTS notifies the MS of the promotions via a Handoff Direction Message. Afterwards, for the MS to commence communication with a new BTS that has been added to the Active Set before terminating communications with an old BTS, a “soft handoff” will occur.
For the reverse link, typically each BTS in the Active Set independently demodulates and decodes each frame or packet received from the MS. It is then up to a switching center or selection distribution unit (SDU) normally located in a Base Station Site Controller (BSC), which is also known as a Radio Network Controller (RNC) using WCDMA terminology, to arbitrate between the each BTS's decoded frames. Such soft handoff operation has multiple advantages. Qualitatively, this feature improves and renders more reliable handoff between BTSs as a user moves from one sector to the adjacent one. Quantitatively soft-handoff improves the capacity/coverage in a WCDMA system. However, with the increasing amount of demand for data transfer (bandwidth), problems can arise.
Several third generation standards have emerged, which attempt to accommodate the anticipated demands for increasing data rates. At least some of these standards support synchronous communications between the system elements, while at least some of the other standards support asynchronous communications. At least one example of a standard that supports synchronous communications includes cdma2000. At least one example of a standard that supports asynchronous communications includes WCDMA.
While systems supporting synchronous communications can sometimes allow for reduced search times for handover searching and improved availability and reduced time for position location calculations, systems supporting synchronous communications generally require that the base stations be time synchronized. One such common method employed for synchronizing base stations includes the use of global positioning system (GPS) receivers, which are co-located with the base stations that rely upon line of sight transmissions between the base station and one or more satellites located in orbit around the earth. However, because line of sight transmissions are not always possible for base stations that might be located within buildings or tunnels, or base stations that may be located under the ground, sometimes the time synchronization of the base stations is not always readily accommodated.
However, asynchronous transmissions are not without their own set of concerns. For example, the timing of uplink transmissions in an environment supporting autonomous scheduling by the individual subscribers can be quite sporadic and/or random in nature. While traffic volume is low, the autonomous scheduling of uplink transmissions is less of a concern, because the likelihood of a collision (i.e. overlap) of data from data being simultaneously transmitted by multiple subscribers is lower. Furthermore, in the event of a collision, there is spare bandwidth available to accommodate the need for any retransmissions. However, as traffic volume increases, the likelihood of data collisions (overlap) also increases. The need for any retransmissions also correspondingly increases, and the availability of spare bandwidth to support the increased amount of retransmissions correspondingly diminishes. Consequently, the introduction of explicit scheduling by a scheduling controller can be beneficial.
However even with explicit scheduling, given the disparity of start and stop times of asynchronous communications and more particularly the disparity in start and stop times relative to the start and stop times of different uplink transmission segments for each of the non-synchronized base stations, gaps and overlaps can still occur. Gaps correspond to periods of time where no subscriber is transmitting. Overlaps correspond to periods of time where multiple subscribers are transmitting simultaneously. Both gaps and overlaps represent inefficiencies in the usage of the available bandwidth and the management of accurate communication.
For example, FIG. 1 is a block diagram of communication system 100 of the prior art. Communication system 100 can be a cdma2000 or a WCDMA system. Communication system 100 includes multiple cells (seven shown), wherein each cell is divided into three sectors (a, b, and c). A BSS 101-107 located in each cell provides communications service to each mobile station located in that cell. Each BSS 101-107 includes multiple BTSs, which BTSs wirelessly interface with the mobile stations located in the sectors of the cell serviced by the BSS. Communication system 100 further includes a radio network controller (RNC) 110 coupled to each BSS and a gateway 112 coupled to the RNC. Gateway 112 provides an interface for communication system 100 with an external network such as a Public Switched Telephone Network (PSTN) or the Internet.
The quality of a communication link between an MS, such as MS 114, and the BSS servicing the MS, such as BSS 101, typically varies over time and movement by the MS. As a result, as the communication link between MS 114 and BSS 101 degrades, communication system 100 provides a soft handoff (SHO) procedure by which MS 114 can be handed off from a first communication link whose quality has degraded to another, higher quality communication link. For example, as depicted in FIG. 1, MS 114, which is serviced by a BTS servicing sector b of cell 1, is in a 3-way soft handoff with sector c of cell 3 and sector a of cell 4. The BTSs associated with the sectors concurrently servicing the MS, that is, the BTSs associated with sectors 1-b, 3-c, and 4-a, are known in the art as the Active Set of the MS. The communication system 100 also provides a message acknowledgment/no acknowledgement (ACK/NACK) procedure by which an active BTS can notify the MS 114 that their last message was not received properly and requires a retransmission or other suitable action.
Referring now to FIG. 2, a communication procedure performed by communication system 100 is illustrated. FIG. 2 is a block diagram of a hierarchical structure of communication system 100. As depicted in FIG. 2, RNC 110 includes an ARQ function 210, a scheduler 212, and a soft handoff (SHO) function 214. FIG. 2 further depicts multiple BTSs 201-207, wherein each BTS provides a wireless interface between a corresponding BSS 101-107 and the MSs located in a sector serviced by the BSS.
When performing a soft handoff, each BTS 201, 203, 204 in the Active Set of the MS 114 receives a transmission from MS 114 over a reverse link of a respective communication channel 221, 223, 224. The Active Set BTSs 201, 203, and 204 are determined by SHO function 214. Upon receiving the transmission from MS 114, each Active Set BTS 201, 203, 204 demodulates and decodes the contents of a received radio frame along with related frame quality information.
At this point, each Active Set BTS 201, 203, 204 then conveys the demodulated and decoded radio frame to RNC 110, along with related frame quality information. RNC 110 receives the demodulated and decoded radio frames along with related frame quality information from each BTS 201, 203, 204 in the Active Set and selects a best frame based on frame quality information. Scheduler 212 and ARQ function 210 of RNC 110 then generate control channel information that is distributed as identical pre-formatted radio frames to each BTS 201, 203, 204 in the Active Set. Alternatively, the BTS of the current cell where the MS is camped (BTS 202) can include its own scheduler and bypass the RNC 110 when providing scheduling information to the MS. The Active Set BTSs 201, 203, 204 then simulcast the pre-formatted radio frames over the forward link. The control channel information is then used by MS 114 to determine what transmission rate to use. Further, the ARQ function is associated with an ACK/NACK channel for use by the BTS to communicate whether the previous message from the MS was received properly by the BTS.
The scheduling function allows a mobile station (MS) to signal control information corresponding to an enhanced reverse link transmission to Active Set base transceiver stations (BTSs) and by allowing the BTSs to perform control functions. The MS in a SHO region can choose a scheduling assignment corresponding to a best transport format and transport-related information (TFRI) out of multiple scheduling assignments that the MS receives from multiple active set BTS. As a result, the uplink channel can be scheduled during SHO, without any explicit communication between the BTSs. In either case, data rate constraints are provided by a scheduler, which is used by the MS 114, along with control channel information, to determine what transmission rate to use.
As proposed for the UMTS system, a MS can use an enhanced uplink dedicated transport channel (EUDCH) to achieve an increased data rate coverage of a reverse link. The MS must determine the data rate to use for the enhanced uplink based on local measurements at the MS and information provided by the UTRAN rate constraints. Moreover, to achieve higher throughput on the reverse link, communication systems such as communication system 100 have adapted techniques such as Hybrid Automatic Repeat ReQuest (H-ARQ) that involves retransmission of erroneous information and Adaptive Modulation and Coding (AMC), as are known in the art.
Adaptive Modulation and Coding (AMC) provides the flexibility to match the modulation and forward error correction (FEC) coding scheme to the average channel conditions for each user, or MS, serviced by the communication system. AMC promises a large increase in average data rate for users that have a favorable channel quality due to their proximity to a BTS or other geographical advantage. Enhanced GSM systems using AMC offer data rates as high as 384 kbps compared to 100 kbps without AMC. Likewise, 5 MHz CDMA systems can offer downlink and uplink peak data rates as high as 10 Mbps and 2 Mbps respectively through AMC, where 2 Mbps and 384 kbps was typical without AMC.
AMC has several drawbacks. AMC is sensitive to measurement error and delay. In order to select the appropriate modulation, the scheduler, such as scheduler 212, must be aware of the channel quality. Errors in the channel estimate will cause the scheduler to select the wrong data rate and either transmit at too high a power, wasting system capacity, or too low a power, raising the block error rate. Delay in reporting channel measurements also reduces the reliability of the channel quality estimate due to constantly varying mobile channel. To overcome measurement delay, a frequency of the channel measurement reports may be increased. However, an increase in measurement reports consumes system capacity that otherwise might be used to carry data.
Hybrid ARQ is an implicit link adaptation technique. Whereas, in AMC explicit C/I measurements or similar measurements are used to set the modulation and coding format, in H-ARQ, link layer acknowledgements are used for re-transmission decisions. Many techniques have been developed for implementing H-ARQ, such as Chase combining, Rate compatible Punctured Turbo codes, and Incremental Redundancy. Incremental Redundancy, or H-ARQ-type-II, is an implementation of the H-ARQ technique wherein instead of sending simple repeats of the entire coded packet, additional redundant information is incrementally transmitted if the decoding fails on the first attempt.
H-ARQ-type-III also belongs to the class of Incremental Redundancy ARQ schemes. However, with H-ARQ-type-III, each retransmission is self-decodable, which is not the case with H-ARQ-type II. Chase combining (also called H-ARQ-type-III with one redundancy version) involves the retransmission by the transmitter of the same coded data packet. The decoder at the receiver combines these multiple copies of the transmitted packet weighted by the received SNR. Diversity (time) gain as well as coding gain (for IR only) is thus obtained after each re-transmission. In H-ARQ-type-III with multiple redundancy, different puncture bits are used in each retransmission. The details for how to implement the various H-ARQ schemes are commonly known in the art and therefore are not discussed herein.
H-ARQ combined with AMC can greatly increase user throughputs, potentially doubling/trebling system capacity. In effect, Hybrid ARQ adapts to the channel by sending additional increments of redundancy, which increases the coding rate and effectively lowers the data rate to match the channel. Hybrid ARQ does not rely only on channel estimates but also relies on the errors signaled by the ARQ protocol.
In the enhanced uplink dedicated channel, the mobile is scheduled by the scheduler, or the mobile can be transmitting in autonomous mode. The BTS sends an ACK/NACK indication to the mobile in response to a message. An ACK (acknowledged) indication acknowledges that the message was properly received. A NACK (not acknowledged) indication indicates that the message was not properly received and should be resent by the MS to the BTS. Optionally, a lack of response from a BTS can be interpreted by the MS as a NACK.
A number of error cases can arise that will degrade the maximum attainable throughput, since an error will generally require a retransmission of the same data. Moreover, the error case of a NACK sent by the BTS being considered an ACK by the mobile can lead to disastrous conditions. In this case, the mobile would have flushed from it's buffer the data packets as soon as an ACK is perceived and this data is lost forever in the case of streaming applications. However, even in non-streaming applications, this type of error would negatively impact system throughput since Receiver-Driven Layered Multicast Congestion Control (RLC) retransmission would be triggered, or in the worst case Transport Control Protocol (TCP) slow start would be triggered, both of which seriously affect data throughput. Additionally, when receiving another scheduling assignment message on the downlink from the BTS, the mobile would now send a different uplink data transmission with contents different from the prior transmission and the BTS would then erroneously soft-combine this in an ARQ operation with information in it's soft buffer that corresponds to previous data, thus affecting the success of transmission of this new packet as well. Solving this problem is therefore important to ensure a high throughput of good data in the uplink. In another error case, the BTS can sent an ACK that the mobile determines to be a NACK, wherein the MS may needlessly retransmit the same data that has already been successfully received at the BTS. This also reduces overall system throughput.
Therefore, a need exists for a new technique to enhance the reliability of the determination of ACK/NACK information, thereby resolving the issues of erroneous determination of an ACK as a NACK and NACK as an ACK, both with and without soft handoff (SHO). In particular, it would be of benefit to set up a technique to allow the feedback of information between the MS and active set BTSs such that a macro selection diversity benefit is obtained.